David Silbersweig: There are circuits in the brain that are evolutionarily conserved, that are quite adaptive under the best of circumstances, that we need, in order to protect us, that are responsible for responding, detecting and responding to threat, generating fear reactions, fight or flight reactions, or the like. And what happens in post traumatic stress disorder is that because of the enormity and severity and immediacy of the traumatic stress, usually life-threatening by definition, that these systems are perturbed and activated in such a way, that they end up causing, in people who are vulnerable, lasting difficulties.

Stephen Smith: Are these parts of the brain that are particularly deep and old?

Silbersweig: They are. In particular, we're interested in the area called the amygdala, which is an area that's important for emotional learning and fear learning. And together with our basic science colleagues, Joe LeDoux and Bruce McEwan, who work on and trace this circuitry in animals with basic science techniques, we actually study them in humans with clinical sorts of techniques.

And in particular, we want to understand the context within which the amygdala operates, namely the circuitry. Other areas, including the hipocampus and the ventral medial, or the lower middle part of the pre-frontal cortex, are particularly important for this as well. In concert, these areas act, so that under the best of circumstances, in an adaptive fashion, we respond appropriately, and detect appropriately, threat, and under the worst of circumstances they can respond inappropriately or in a prolonged or intermittent fashion and produce the symptoms of PTSD.